When you speak of the collapse of a civilization people imagine either a sudden, violent end, like Pompeii, all life snuffed out in an instant, or a gentle decline with the population dwindling toward poverty and genetic unsustainability.
The reality is much grimmer. Evidence indicates that civilizations collapse rather suddenly, but not instantly. Several systems which had been providing for the basic needs of the population abruptly fail. The result is a drawn out, violent scramble for basic resources – food, water, energy, etc. In the Mayan collapse 90% to 99% of the population disappeared. The remainder of the population fled into the wilderness and was absorbed by other groups. In most cases the systems had been pushed to the tipping point and some outside event, like drought, disease, or invasion crashed the systems. In some cases, the Anasazi for example, larger groups left when the collapse became inevitable. They moved out into the wilderness, merging with other, simpler societies and survived.
For us, fleeing to the wilderness isn’t a viable option. There isn’t enough wilderness left to actually support viable populations, and there are no remaining societies we could join. The collapse of a global civilization like ours would result in waves of violent competition for ever scarcer resources, cascading through global population centers until humanity was diminished to the point that the remaining people could survive on the simple resources they could find. Perhaps humanity would survive, perhaps not.
So, where are we on the path to collapse? How close to the tipping point are the systems that provide our food, water, energy? What happens if one of these basic systems collapses?